Method of making wave guides



United States Patent 3,341,432 METHOD OF MAKING WAVE GUIDES WoldemarKadner, Birkenfeld, Wurttemberg, Germany,

assignor to International Standard Electric Corporation, New York, N.Y.,a corporation of Delaware No Drawing. Filed Mar. 6, 1964, Ser. No.349,981 Claims priority, application Germany, Mar. 14, 1963,

5 Claims. (Cl. 204-99) ABSTRACT OF THE DISCLOSURE A wave guide iselectro-formed by depositing metal on a lost mandrel which is thenmelted away, leaving the deposited shell. The interior of the shell isflushed with aquaregia, and other chemicals. The mandrel is made in anegative pressure atmosphere to improve the fineness of its surfacefinish.

This invention relates to methods of making wave guides and moreparticularly to methods of electroforming wave guides.

Generally, wave guides may be made by any number of well knownprocesses, one of which is herein termed electroforming. By this term, Imean that the wave guide is formed by depositing material upon amandrel, as by a plating or galvanizing-like process. Then the depositedmaterial is removed from the mandrel to leave the shell of plating-whichis the wave guide.

One way of electroforming involves the use of a permanent, high grade,well made steel mandrel on which the wave guide may be deposited 'by theelectroforming process. This permanent mandrel process is very goodbecause it gives not only an excellent surface on the inside of the waveguide, but also insures a precise size and shape. Unfortunately,however, the shell of plating material must be slipped off the permanentmandrel. This is not too hard to do if the mandrel is in the form of asmooth and simple shape. However, if the mandrel is a complicated shape,full of undercuts, recesses, protrusions,

.and the like, it may not be possible to slip the shell of platingmaterial off the mandrel.

Another Way of electroforming wave guides involves the use of a lostmandrel which is destroyed during the electroforming process. Forexample, if a mandrel is made from a material having an extremely lowmelting point, it may be used to support the shell of plating materialduring the electroforming process. Then the mandrel may be melted away,thereby leaving only the shell of plating, which is the wave guide. Thisway almost any complicated shape may be formed. The invention relates tothis lost mandrel process.

In the past, lost mandrels have been made of a metal alloy which meltsat an extremely low temperature, such as an alloy which consists ofbismuth-lead-tin-indium. However, experiments show that satisfactoryresults are not always possible with such a mandrel material for thefollowing reasons:

(1) The bismuth-lead-tin-indium alloy sometimes combines itself with theplating material. Therefore, the mandrel cannot be completely removedfrom the shell of plating material by a melting process. A mechanicalcleaning of the inside of the plating shell is neither economioally nortechnically practical. Also, it is difficult to use a preliminarytreatment-such as applying a plastic coating to the mandrel-because thiscoating would unpredictably change the size and dimensions of thevarious components.

(2) The only practical way of making a lost mandrel is by a moldingprocess. But, this process often causes bubbles, cavities, and otherirregularities which form on the surface of the lost mandrel due to aninsuflicient wetting of the mold resulting from the relatively highsurface tension of the liquid metal alloy.

In keeping with one aspect of the invention, these and otherdisadvantages are eliminated by a method of manufacture which enablesthe use of a conventional bismuthlead-tin-indium a-lloy as a lostmandrel material. First, the mandrel is made by molding it under anatmosphere having a negative pressure. During the electroforming of awave guide, several layers of different kinds of plating material aredeposited onto the mandrel whereby the interior and the exterior layersare silver and an intermediate layer is a base metal. The lost mandrelis then melted and its residues are eliminated by flushing with aquaregia. Since aqua regia dissolves almost every metal except silver, thewave guide is freed of all traces of the lost mandrel without damage tosurface conditions.

A pressure chamber or box represents any suitable way of providing anegative atmosphere pressure. While any suitable means may be provided,I use a simple closed chamber which is connected to a vacuum pump. Inone embodiment, the pump is able to evacuate the pressure chamber to anegative pressure of about a 10 mm. mercury-column. Any suitable inputis arranged to convey a molten alloy having a low melting point into thenegative pressure chamber through a valve-like arrangement. The valve isany suitable device for allowing entry of the molten alloy without lossof the negative pressure.

Inside the negative pressure chamber is a mold for making the lostmandrel. The extremely simple mold comprises a cope and a drag, joinedtogether by any suitable means. The cope contains a pair of smalldrilled holes (perhaps 5 mm. in diameter) forming an ingot and a risergate. When the cope and drag are fastened together, they form a matrixin which the lost mandrel is formed. This matrix may have anygeometrical shape.

The ingot is positioned adjacent the input in a manner such that themolten alloy flows through the input ingot, fills the matrix and raisesinto the riser. The negative atmospheric pressure in the chamber insuresa smooth surface on the lost mandrel free of bubbles, cavities, andother irregularities.

The mold is made of brass. The inside surfaces of this mold must beprepared with sufficient care to meet the exacting requirements of awave guide surface quality, size, and dimensions. Furthermore, the moldmust consist of as many individual parts as is necessary for a properremoval of the lost mandrel after it has hardened in the mold. Brass isused as mold material because it is relatively simple to fabricate anddoes not combine chemically with the lost mandrel material. Thus, themold does not require any protective coat which would make it difiicultto meet the extremely precise wave guide requirements as to size anddimension. A further advantage of brass is that it does not oxidize whenthe mold is cooled down.

Since the lost mandrel is made from an alloy having a very low meltingpoint (below centigrade), the thermal, linear expansion or contractionof the brass mold is of minor importance. Moreover, when the mold ismade, allowance may be provided to correct any small dimensional changeswhich do occur as a function of temperature.

After it has hardened in the matrix, the lost mandrel is removed fromthe mold by separating the cope and the drag. At this time the residueof the ingot and riser may be broken off the mandrel, and any resultingsurface irregularities are removed from the mandrel, as by grinding orfiling. Since the mandrel can be molded or dieeast in its final size, itcan be used for electroforming of wave guide components without furthertreatment.

The wave guide component is electroformed onto the 3 lost mandrel in anywell-known manner, as by plating, for example. To obtain a smoothsurface, the lost mandrel is moved in its electrolytic plating bathtoward and away from the plating electrode.

As already mentioned, the lost mandrel will be melted out of the shellof plating material, and the residues of the mandrel material remainingon the inside surface of the plating shell will be dissolved by aquaregia. However, since aqua regia dissolves nearly all metals, exceptsilver, the entire shell of plating must be embedded in a thin silverlayer. This layer is obtained by plating the shell in successive layersof different kinds of metal. Therefore, the lost mandrel is first platedin a bath with a suitable silver solution to provide an inside layer ofsilver. T hereafter, the wave guide component receives its prescribedwall thickness of about 1.7 mm. in a bath of base material, such as asuitable copper solution. Finally, the wave guide is again silver-coatedin the silver solution bath. This way, the lost mandrel can be meltedout of the wave guide, and its residues can be eliminated by flushingwith aqua regia, heated up to approximately 50 centigrade. Since thebase metal is entirely embedded in silver, the aqua regia bath may beused without damaging the wave guide component.

When producing smaller components withless metallic volume, theproduction costs caused by the multi-layer plating process may not bejustified. Thus, a solid silver plating shell is recommended.

Due to the aqua regia treatment, an oxide layer is caused at the waveguide components. This layer is finally flushed oiT with hydrochlorid,heated up to 50 centigrade.

While the principles of the invention have been described above inconnection with specific apparatus and applications, it is to beunderstood that thi description is made only by way of example and notas a limitation on the scope of the invention.

I claim:

1. The method of manufacturing wave guides comprising the steps ofmolding a lost mandrel under negative atmospheric pressure from amaterial having a low melting point, said material being soluble in aquaregia, electroforming a wave guide on the lost mandrel by first platingthe mandrel with a layer of silver, then with a layer of base met-a1,and following with another layer of silver whereby said base metal isencased in silver, melting the lost mandrel out of the shell of platingmetal to leave a wave guide, and flushing theinterior of the wave guidewith aqua regia to eliminate all residue of the lost mandrel.

2. The method of claim 1 including a final step of cleaning the interiorof the wave guide with concentrated hydrochloric acid.

3. The method of claim 1 wherein said aqua regia is heated toapproximately centigrade.

4. The method of claim 2 wherein said hydrochloric acid is heated toapproximately 50 centigrade.

.5. The method of claim 1 wherein said lost mandrel is made from analloy of bismuth-lead-tin and indium.

References Cited UNITED STATES PATENTS 1,320,770 11/1919 Lougheed 266342,490,193 12/1949 Barr 266-34 2,592,614 4/1952 Stoddard 204-9 OTHERREFERENCES Publicationi Safranek, W. H. Schickner, W. C., and Faust, C.L., Electroforming Aluminum Wave Guides Using Organo-Aluminum PlatingBaths, Journal of the Electrochemical Society, vol. 99, No. 2, February1952,

JOHN H. MACK, Primary Examiner. HOWARD S. WILLIAMS, Examiner.

D. R. VALENTINE, Assistant Examiner.

1. THE METHOD OF MANUFACTURING WAVE GUIDES COMPRISING THE STEPS OFMOLDING A LOST MANDREL UNDER NEGATIVE ATMOSPHERIC PRESSURE FROM AMATERIAL HAVING A LOW MELTING POINT, SAID MATERIAL BEING SOLUBLE IN AQUAREGIA, ELECTROFORMING A WAVE GUIDE ON THE LOST MANDREL BY FIRST PLATINGTHE MANDREL WITH A LAYER OF SILVER, THEN WITH A LAYER OF BASE METAL, ANDFOLLOWING WITH ANOTHER LAYER OF SILVER WHEREBY SAID BASE METAL ISENCASED IN SILVER, MELTING THE LOST MANDREL OUT OF THE SHELL OF PLATINGMETAL TO LEAVE A WAVE GUIDE, AND FLUSHING THE INTERIOR OF THE WAVE GUIDEWITH AQUA REGIA TO ELIMINATE ALL RESIDUE OF THE LOST MANDREL.